1. Field of the Invention
The invention relates generally to a control unit and method for a vehicle equipped with a stepped automatic transmission. More specifically, the invention relates to such control unit and method that prevents shift shock from being caused during drive power demand control.
2. Description of the Related Art
A control technique called “drive power control” is often employed in a vehicle equipped with an automatic transmission and an engine which is controllable to output a required torque independently of an accelerator pedal operation performed by a driver. According to the drive power control, a positive or negative target drive power is calculated based on the amount by which the accelerator pedal is operated by the driver, the operating conditions of the vehicle, and the like. Based on the calculated target drive power, the engine torque and the gear ratio of the automatic transmission are controlled. Examples of the drive power control include “drive power demand control” and “torque demand control”.
In this drive power control, the target drive power for the vehicle is calculated based on the vehicle speed and the accelerator pedal operation performed by the driver. Then, the gears (gear ratio) and the engine torque are controlled to achieve the target drive power. The engine torque is set using a map indicating the throttle valve opening amount required for outputting the target drive power set for each gear. When the gears are changed, the time, at which the gears are actually changed (the time at which gear-change is actually started), and the time, at which the throttle valve opening amount is changed, are controlled to be synchronized with each other. In this case, responses of the transmission and the throttle valve to the controls are taken into account. Thus, shift shock is reduced.
Japanese Patent Application Publication No. 2001-347854 (JP-A-2001-347854) describes a drive power control unit for a vehicle equipped with a stepped automatic transmission, which reduces shift shock. This drive power control unit includes first means for calculating the target drive power; second means for calculating the target engine torque based on the target drive power and the gear ratio; and third means for performing control so that the gear ratio of the current gear is used to calculate the target engine torque when the gears are not being changed, and the actual gear ratio, calculated based on the rotational speeds of the input shaft and the output shaft of the transmission, is used to calculate the target engine torque while the gears are being changed.
The drive power control unit performs such control when the gear ratio is changed. Accordingly, with this drive power control unit, the target engine torque is gradually changed based on the actual gear ratio while the gears are being changed. As a result, it is possible to prevent sudden reduction in torque, and thereby to reduce shift shock. However, if the actual gear ratio calculated based on the rotational speeds of the input shaft and the output shaft of the transmission is used, shift shock may be increased by sudden increases in the engine torque due to changes in the calculated value of the actual gear ratio. Such changes occur due to a malfunction in a sensor or disengagement of a one-way clutch. However, the drive power control unit described above uses the actual gear ratio only while the gears are being changed, and the gear ratio of the current gear is used when the gears are not being changed. Therefore, it is possible to prevent increases in shock given to the vehicle. While the gears are being changed, friction elements of the transmission are slipping. Accordingly, even if the engine torque is suddenly increased, the increase in the engine torque is not entirely reflected on the torque output from the transmission. For example, only the torque within the capacity of a clutch, one of the friction elements, is transferred, and the remaining torque just contributes to increases in slippage of the clutch. Accordingly, although shock is temporarily caused, such shock is absorbed in the slippage of the clutch. Moreover, even while the gears are being changed, until the gear ratio starts changing due to start of the inertia phase, the gear ratio of the gear is used instead of the actual gear ratio. With such control, the target engine torque is not changed at the initial stage of the inertia phase, but is changed after the gear-change has proceeded to a certain degree in the inertia phase. Thus, it is possible to prevent increases in shift shock, delay in a change in the target engine torque, and the like.
The drive power control unit described above calculates the torque to be output from the internal combustion engine, using the gear ratio of the current gear when the gears are not being changed, and using the actual gear ratio calculated based on the rotational speeds of the input shaft and the output shaft of the transmission (=rotational speed of input shaft of transmission (rotational speed of turbine)/rotational speed of output shaft of transmission) while the gears are being changed.
Usually, a transmission is provided with a one-way clutch that transmits drive power only in one direction. When the torque to be output from the engine is calculated in the above-described manner, if the one-way clutch is disengaged, the gear ratio of the gear deviates from the actual gear ratio. In such a state, if the gear ratio used to calculate the target engine torque is changed, the engine torque may suddenly changes, and, consequently, shock may be caused. If the engine torque is controlled using the actual gear ratio while the gears are being changed, the engine torque may not be performed stably if the rotational speeds fluctuate or the detection accuracy of the sensor is not sufficiently high.
However, in the control of the torque output from the engine, which is the drive power source of the vehicle, described in Japanese Patent Application Publication No. 2001-347854 (JP-A-2001-347854), such inconveniences are not taken into account.